This invention relates to transponders, devices which, upon receiving a predetermined interrogate signal, respond by transmitting desired information to a receiver. More particularly, the invention is directed to passive transponders for use with acoustic surface waves.
Transponders are conventionally used to receive an interrogating signal or signals and then to transmit a response. For example, such devices are widely employed in commercial and military aircraft. Typically, an air traffic controller transmits an interrogation signal to the aircraft-carried responder which then automatically responds giving information such as flight number and altitude. This information enables the ground station to distinguish between and to identify aircraft. Transponder systems are also employed for vehicle or personnel detection and identification.
Existing transponders systems are costly to produce, especially where a delay signal is required. Such signal delay enables one to discriminate the desired signal from the reflected radio waves, clutter or background.
Passive transponders can also discriminate against reflected radio waves without using signal delay. For example, non-linear diodes can be used for discriminating clutter signals from the desired signal. In operation, the non-linear diodes receive a predetermined signal at one frequency causing the generation of other frequencies, or harmonics, which may be distinguished from the original signal and background clutter by virtue of frequency. While the non-linear diode signal discriminators are inexpensive to make, they are very inefficient in converting energy into other frequencies. Moreover, this method of signal discrimination is impaired by generation of similar harmonics in the transmitter and receiver and in other objects containing non-linear scattering mechanisms. It is, accordingly, the aim of the present invention to provide an improved passive responder which avoids the deficiencies and shortcomings of prior art systems.